Apparatus for applying an electric field

ABSTRACT

The present invention teaches an apparatus for producing an article in a mold tool having electrically isolated dies, an electric field generator, and a sleeve made from a material having a very low dielectric loss factor and being operable between an extended and retracted position. The sleeve separates the dies when it is in an extended position and permits an electric field produced by the generator to be coupled between the dies.

BACKGROUND OF THE INVENTION Cross-Reference to Related Application

Copending patent application, by the same applicant and assigned to thesame assignee, discloses related subject matter, entitled "In-MoldPlasma Treatment", U.S. Ser. No. 07/772,187, filed Oct. 7, 1991.

Field of the Invention

The present invention relates to an apparatus for coupling an electricfield between opposed dies of a tool. More specifically, the inventionrelates to an apparatus for creating a plasma in an injection moldingtool to make the molded article more receptive to coatings or to apply acoating on the article.

Description of Related Art

It is known to expose articles to a plasma to change the surfacecomposition of the article. Such exposure is generally termed "plasmatreatment". Low pressure plasma treatment is useful for stabilizing amolded article containing a plasticizer, and for making the article morereceptive to coatings such as paints, lacquers, adhesives, films, etc.It is also useful to plasma-treat an article for antistatic purposes.Generally, low pressure plasma treatment involves placing the article tobe treated in a chamber and reducing the chamber pressure to between1-0.01 Torr. An ionizable gas is introduced into the chamber and the gasis transformed into a plasma. One method of creating the plasma is bypassing the gas through an electric field created between twoelectrodes. The electric field excites the gas and ionizes theindividual molecules of the gas to form a plasma. The plasma treats allexposed surfaces of the articles within the chamber. This method oftreating one or more articles in a chamber is known as batch processing.The chamber must be stocked with articles for plasma treatment, and thenthe entire chamber evacuated to a relatively low pressure. The largerthe chamber, the longer the evacuation time and more expensive andcomplex are the individual components necessary to draw vacuum andcreate a plasma. One such example of a batch processing plasma treatmentis taught and described in U.S. Pat. No. 4,107,049 issued to Sano et al.

Surface modification, specifically for polymeric substrates, is usefulfor enhancing the adhesion properties of the substrate. Polymericmaterial, such as polypropylene and polyethylene, contain fullyhydrogenated surfaces which are not polar and therefore not receptive tocoatings. By exposing this surface to a plasma, various constituentmolecules having more polar species can be grafted onto the polymer.These polar species enhance the adhesion of the surface to variouscoatings. Previous attempts of grafting adhesion-promoting moleculesonto a substrate required batch or semi-batch processing. Batchprocessing has previously been described as taught in Sano et al.Semi-batch processing consists of a series of chambers, each containingarticles to be treated. The chambers are evacuated in series. The plasmais produced by a plasma generating means outside of the chambers. Oncean individual chamber has been evacuated to the proper pressure, theplasma is introduced to treat the articles therein. Semi-batchprocessing permits the plasma to be created in one location while thechamber containing the article to be treated is evacuated. Semi-batchprocessing permits a slight increase in speed over batch processing.Because both batch and semi-batch processes use chambers having a volumefar in excess of that required to create a plasma, these processes arestill relatively inefficient.

It is also known to create an electric field between opposed dies fordielectric heating of an article placed between the dies, as taught inU.S. Pat. No. 4,134,942 issued to Mirr et al. Mirr et al teaches usingelectrically nonconductive mold sections, each incorporating a separateelectrode. The mold sections are closed and a foamable material isintroduced between the mold sections and a radio frequency (RF) electricfield is applied between the electrodes. The RF electric field heats thefoamable material and accelerates cure. Mirr et al teaches the use ofnonconductive dies so as to electrically isolate them. This permits anelectric field to be coupled between the dies when closed.

It is an object of the present invention to provide an apparatus forcoupling an electric field between opposed dies made from a conductivematerial.

It is another object of the present invention to provide an apparatusfor producing a plasma to treat a molded article.

It is still a further object of the present invention to provide aninjection molding tool which also treats the freshly molded article witha plasma.

SUMMARY OF PREFERRED EMBODIMENTS

The present invention teaches an apparatus for producing articles in amold tool, comprising electrically isolated dies; an electric fieldgenerator; a sleeve made from a material having a very low dielectricloss factor and being operable between an extended and retractedposition. The sleeve separates the dies when it is in an extendedposition and permits an electric field produced by the generator to becoupled between the dies.

The dies are electrically isolated from one another. They may beelectrically conductive and spaced apart by an insulator. In thealternative, the dies themselves may be made from a dielectric materialhaving a low dielectric loss factor such as a ceramic. If an insulatoris not used, only one die need be electrically nonconductive. The otherdie may be electrically conductive or electrically nonconductive. Whenusing either one or more electrically nonconductive dies, separateelectrodes are placed within or adjacent the dies to couple the electricfield. Conductive dies do not require a separate electrode andthemselves act as an electrode.

The apparatus of the present invention may be used for applicationsrequiring an electric field between opposed dies. Applications includedielectric heating, orientation of polar molecules, and plasmatreatment. These as well as other uses of the present invention areenabled based on the present disclosure and covered by the attachedclaims.

The preferred embodiment of the present invention will is an apparatusfor creating a plasma in a molding tool. The tool has electricallyisolated dies separated by a sleeve. The sleeve spaces the dies apart adistance when the sleeve is in an extended position. When the apparatusis used for plasma treatment, the space adjacent the article isevacuated and an ionizable gas is introduced. A plasma is created bycoupling an electric field in the space containing the gas. The gas istransformed into a plasma and treats exposed surfaces within the space.Various ionizable gases are suitable for plasma treatment. Selecting theappropriate ionizable gas is dependent upon the type of molecule to besubstituted and the surface of the article to be treated. In the case ofadhesion promotion, molecules such as O₂, CO₂, CO, NO₂, SO₂, NH₃, SF₆,N₂, and many others, either separately or in combination, form a plasmawhich bonds well to polymeric materials. The most common and leastexpensive adhesion promoting gas useful for plasma treatment is air.

Plasma polymerizable coatings are also useful for treating articles. Toform a plasma polymerized coating on an article, a polymerizable monomersuch as hexamethyldisiloxane, ethylene, methymethacrylate, as well asmany others, is introduced into the chamber. An electric field isapplied to the gas and a plasma created. The plasma polymerizes onto thesurface of the article. Polymerizable coatings are useful for a varietyof functions, including permeation resistance, durability, and abrasionresistance.

A plasma is created when a high frequency electric field is applied toan ionizable gas under low pressure. This electric field is preferablyapplied by connecting one terminal of an RF generator through animpedance matching network to the molding equipment. Both dies areelectrically conductive, but one die is electrically isolated from themolding equipment. This die is connected to one terminal of the RFgenerator. The other die is conductive with the molding equipment and isconnected to the ground terminal of the RF generator. The diesthemselves act as electrodes for the RF generator. The generator appliesan electric field between the dies. The electric field creates a plasmafrom the gas within the space. A molded article between the dies isexposed to the plasma and free radicals formed by the electric field aresubstituted on the surface of the article. To avoid a buildup of ions orelectrons adjacent the electrodes, an alternating electric field isused.

When the electric field is withdrawn, the plasma returns to a gaseousstate and the article is removed from the mold. The surface of themolded article has been altered by substituting free radicals from theplasma onto the article surface. When the article is made from apolymeric material, this surface treatment raises the Gibbs free energyof the surface. To lower the Gibbs free energy, the substrate oftenrotates the surface polymeric chains to encase the substituted moleculeswithin the substrate. The substrate surface attempts to return to itshydrogenated, low polar character and reduces the adhesion properties ofthe substrate. To avoid this rotation, it is preferable to coat theplasma treated article before it has lowered its Gibbs free energythrough internalization of the substituted radicals.

The present invention provides an apparatus for manufacturing a plasmatreated article by creating a plasma in a small volume space. By usingelectrically isolated mold dies as the electrodes, separate vacuumchambers, and plasma-generating vessels are eliminated. Creating aplasma after an article has been freshly molded reduces the cycle timefor plasma treatment and permits the plasma treatment of an articlewhile still in a semi-molded and heated state. Separate reheating of thearticle to promote susceptibility to plasma treatment is unnecessary.

Another feature of the present invention providing an apparatus forisolating conductive dies to couple an electric field therebetween.

Still another feature of the present invention is to provide a spaceadjacent an article and applying an electric field across this space.

These and other objects, features, and advantages of the presentinvention will be more fully described with reference to the followingdetailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a mold tool having electrically isolateddies.

FIG. 2 is a sectional view of the mold illustrated in FIG. 1 showing theinjection molding of an article.

FIG. 3 is a sectional view of the mold illustrated in FIG. 1 showing theplasma treatment of the freshly molded article.

FIG. 4 is a sectional view of the mold illustrated in FIG. 1 in the openposition showing the plasma treated article being removed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment of this invention, an injection moldingapparatus is provided. Other molding apparatuses, including compressionmolding, low pressure compression molding, stamping, and blow moldingare also possible using the invention taught herein.

The preferred embodiments include the plasma treatment of a polymericsubstrate and an apparatus using conductive dies and coupling anelectric field across a space adjacent an article. The space adjacentthe article may be partially evacuated or filled with a fluid. A plasmamay be created in this space to treat an article. Electricallyconductive substrates may be plasma treated so long as they areelectrically isolated from at least one mold die which is acting as anelectrode so that a potential may be created in the space adjacent thesubstrate.

The invention will be described as an apparatus for the plasma treatmentof an injection molded article as shown in FIGS. 1-4. FIG. 1 shows mold10 having movable dies 12, 14. Dies 12, 14 have matched surfaces formingcavity 16 when closed. Cavity 16 provides the shape for molded article18 shown in FIGS. 2-4. Dies 12, 14 are shown in the closed position inFIG. 1. Shoulder 20 of die 12 rests against shoulder 21 of die 14 whenthe mold is in the closed position. Sprue 22 communicates with cavity 16and supplies a thermoformable polymeric material for molding article 18.A gate may be added to the end of sprue 22 to further direct the flow ofmaterial into cavity 16.

Die 14 is secured to base 24. Base 24 and die 14 remain stationaryduring the molding operation. Base 24 contains a raised portion 26having a smaller diameter than die 14. Located intermediate base 24 anddie 12 is sleeve 28. Sleeve 28 appears L-shaped when viewed incross-section. Sleeve 28 acts as an electrically nonconductive isolator,spacing dies 12, 14 apart. Sleeve 28 is attached to pistons 30 and isoperable to raise die 12 from die 14. Sleeve 28 contains seals 32, 33.Die 12 is secured to dielectric isolator platen 34 through the use ofnonconductive connectors (not shown). Platen 34 and sleeve 28 arepreferably made from an electrically nonconductive material having avery low dielectric loss factor at radio frequencies. Particularlypreferred as materials for the isolator platen are ceramics such as T-Tzirconia because of its high compressive strength and fracturetoughness. The sleeve can be formed from a ceramic material or from apolymeric material such as ultra-high molecular weight polyethylene.

Mold 10 is attached within hydraulic press 36. Hydraulic press 36 is ofthe type generally used for injection molding operations and may apply aforce up to approximately 4000 tons. Hydraulic pressure from press 36may also be diverted to pistons 30 to assist in raising and loweringsleeve 28. Pistons 30 may be alternatively operated by auxiliaryhydraulic, pneumatic, or mechanical means.

Die 12 contains vacuum channel 38 and gas channel 40. Channels 38, 40communicate with shoulder 20 and are normally sealed by shoulder 21 whenthe dies are in the closed position. Alternatively, channels 38, 40 maybe placed within stationary die 14 to avoid using flexible connectorsfor gas and vacuum lines.

Pistons 30 are retracted, lowering sleeve 28 as shown in FIGS. 1 and 2.Dies 12, 14 are closed and a pressure of approximately 4000 tons isapplied to the dies by press 36. The amount of pressure applied by press36 is dependent on molding temperature, material, mold shape, andarticle thickness. Article 18 is molded within mold 10 by injecting athermoformable material through sprue 22 into cavity 16. Thethermoformable material flows throughout cavity 16 and takes the shapeof molded article 18. The thermoformable material does not pass betweenshoulders 20, 21. After molded article 18 has cooled for a sufficientperiod of time that exterior pressure on the article from dies 12, 14 isno longer necessary, the clamping tonnage applied by press 36 isreduced. Pistons 30 are moved to their extended position and raisesleeve 28 as shown in FIG. 3. Seal 32 contacts die 12 and forms anair-tight seal between sleeve 28 and die 12. Pistons 30 apply a greaterforce on die 12 than the opposing force applied by press 36. Thispressure differential assures that seal 32 seals against die 12 duringthe die separation process. Continued force from pistons 30 raises die12 from die 14. Seal 33 engages the lower portion of die 14 and forms anair-tight seal between die 14 and sleeve 28. Pistons 30 need only applya slightly greater pressure than press 36 to raise die 12 from die 14and continually maintain a seal. Sleeve 28 is constrained fromadditional upward movement by the lower face 42 of die 14.

The displacement of die 12 forms a space 44, as shown in FIG. 3. Space44 is bounded on its upper surface by die 12 and on its lower surface byarticle 18 and die 14. Sleeve 28 seals the periphery of space 44.Shoulder 20 no longer engages shoulder 21, permitting channels 38, 40 tocommunicate with space 44. Valve 46 connects vacuum pump 48 with vacuumchannel 38. Vacuum pump 48 evacuates space 44 to a Pressure between0.1-1 Torr. Flow control valve 50 permits the introduction of anionizable gas into space 44 at a predetermined rate. The ionizable gasis drawn into space 44 by the vacuum created by pump 48. It is desirableto continue bleeding a quantity of ionizable gas into space 44throughout the plasma treatment operation. The plasma treatment depletesthe ionizable gas. Additional gas is introduced to maintain the plasma.

Selecting the appropriate ionizable gas is dependent upon the type ofchemical functionality to be substituted on the surface of moldedarticle 18. In the case of a polymeric article, gases such as O₂, CO₂,NO₂, air, and others, separately or in combination, are useful forpromoting adhesion of coatings by the incorporation of polar functionalgroups. It has been found that these low-cost gases, or even air, cancreate a suitable plasma for plasma treatment of polymeric materials foradhesion enhancement. These ionizable gases are supplied to flow controlvalve 50 and introduced at a rate of approximately between 20-60 SCFM. Alow pressure of approximately 0.1-1 Torr is maintained throughout theplasma treatment by vacuum pump 48.

Die 12 is connected to one terminal of impedance matching network 52.Base 24 is connected to the ground terminal. Network 52 is attached toRF generator 54. Generator 54 and network 52 are generally commerciallyavailable systems typified by model RFX-600 Generator and ATX-600 Tuner(matching network) manufactured by Advanced Energy Industries, Inc.,Fort Collins, Colo. Generator 54 produces an electric field at afrequency of 13.56 MHz and at a variable power level from 0-600 watts.Network 52 matches the impedance of mold 10 in the extended sleeveposition to permit complete coupling of the electric field densitybetween dies 12 and 14.

Base 24 and die 14 are electrically conductive and may be made from toolsteel. Die 12 is also electrically conductive and may be made from toolsteel, but is electrically isolated from die 14 and base 24 via isolatorplaten 34 and sleeve 28. Dies 12, 14 act as electrodes for coupling theelectric field in space 44. An electric field of 13.56 MHz (or multiplesthereof) is applied to space 44. The Boltzmann distribution functionpredicts that a certain number of molecules or atoms of gas will existas ions. These ions are excited by the electric field to increase theenergy of the gas. The elevated energy of the gas produces a dielectricbreakdown of the gas until a plasma is created in space 44. The plasmatreats all exposed surfaces of article 18. Article 18 may be lifted fromdie 14 to expose additional surfaces to plasma treatment if desired (notshown).

The plasma treatment may comprise a surface substitution of atoms ormolecules of the ionizable gas or a plasma polymerization of monomersonto the surface of article 18. In some instances, the elevatedtemperature of recently molded article 18 assists in the plasmatreatment. Additional heating or cooling elements within dies 12, 14 maybe used to tailor the plasma treatment to the particular substrate andplasma gases.

In the preferred embodiments of the invention, an article manufacturedfrom rubber-modified polypropylene (TPO) is exposed to a plasma formedfrom air. After an exposure of approximately 10-30 seconds, the electricfield is withdrawn. While not wishing to be bound by the followingtheory, it is believed that a majority of the ion substitution or plasmapolymerization occurs after the withdrawal of the electric field. Thisis believed due to free radicals in the plasma combining with activesites on the surface of the article. To promote this effect and alsoreduce the overall time required for the plasma treatment, the electricfield may be pulsed using the pulsing circuit taught and described inU.S. application Ser. No. 07/633,742, filed Dec. 24, 1990, entitled"Dielectric Curing of Adhesives", incorporated herein by reference.Pulsing is defined as rapidly applying and withdrawing the electricfield. Pulsing has the effect of rapidly creating and withdrawing aplasma in space 44. It is believed that the rapidly creating andwithdrawing the plasma will promote plasma treatment for a definedperiod of time.

After sufficient plasma treatment of the molded article, sleeve 28 isretracted and mold 10 opened. Article 18 is removed from mold 10. Plasmatreatment of polymeric material such as TPO for use as adhesionpromotion does not remain effective indefinitely. Functional groupssubstituted onto the surface of the polymeric chains of the TPO tend tointernalize via rotation to reduce the Gibbs free energy of the surface.The polar species grafted onto the polymer to promote adhesion are nolonger exposed on the surface. When an article is exposed to plasmatreatment for adhesion enhancement, it is preferable that -the articlebe painted or bonded within 20-30 days after the plasma treatment. Thislength of time may vary depending upon the substrate material, type ofplasma treatment, amount of surface substitution, and type of coating.

While the invention has been described in its preferred embodiments asan apparatus for the plasma treatment of an injection molded article,various modifications and changes may be made without departing from thespirit and scope of the invention. The apparatus taught herein may beused with compression molding, stamping, or blow molding devices. Ineach of these alternative methods, electrically isolated dies areseparated with a sleeve made from a material having a very lowdielectric loss factor and permit an electric field to be coupled inbetween the dies. These as well as other changes and modifications tothe preferred embodiments are intended to be included within the scopeof the attached claims.

I claim:
 1. An apparatus for producing a plasma treated article betweenopposed dies, comprising:first and second dies made from an electricallyconductive material and molding said article therebetween, said firstand second dies being electrically isolated from one another; a ratiofrequency generator connected between said first and second dies; asleeve made from a material having a very low dielectric loss factor,said sleeve encircling said first die and having a means for sealingagainst said first and second dies when said sleeve is in an extendedposition; means for operating said sleeve between said extended positionand a retracted position, said sleeve separating said first and seconddies a distance and forming a space between said article and at leastone said die when said sleeve is in said extended position; means forcreating a vacuum in said space; and means for introducing an ionizablegas in said space whereby an electric field produced by said generatoris coupled in said space and said ionizable gas forms a plasma whichtreats all exposed surfaces of said article.
 2. The apparatus of claim1, further comprising means for injection molding said article betweensaid first and second dies.
 3. The apparatus of claim 1, furthercomprising an impedance matching means connected between said generatorand said first and second dies for matching the impedance of said firstand second dies while said sleeve is in said extended position with theimpedance of said generator.
 4. The apparatus of claim 1, furthercomprising means for pulsing said electric field.